Renewables

Why Renewables? The Pros

• Reduced emissions
  
• Decreased fossil fuel dependence
  
• Consumer support
  

Reduced Carbon Emissions

An international scientific panel concluded, "human activities are changing the atmospheric concentrations and distributions of greenhouse gases and aerosols." The single largest source of such gases is electricity generation. According to an Environmental Protection Agency report, electricity generation is responsible for 34 percent of all U.S. greenhouse gas emissions – more than transport (27 percent), industry (19 percent), or any other economic sector. Several gases contribute to climate change, however the single greatest contribution comes from CO2. Electricity generation is responsible for 40 percent of total U.S. CO2 emissions. Fuels used for electricity vary quite a bit in their carbon intensity (Table).

Table: Carbon intensity of electricity generating fuels

Source: EPA

Most forms of renewable energy, in contrast, have zero or near-zero carbon emissions. Using renewable electricity that displaces fossil-based electricity results in direct and measurable reductions in CO2 emissions. The amount of these emissions reductions depends on the type of renewable used, and the fossil-based electricity it displaces. Wind and solar have essentially zero carbon emissions. Using 1 kWh from wind to replace 1 from coal, for example, saves 2.1 pounds of CO2. Hydropower has essentially zero carbon per kWh of output, but some argue that building dams results in CO2 emissions due to the inundation of biological matter. Closed-loop biomass (in which plants are grown specifically for burning) is generally viewed as resulting in no additional carbon emissions, while open-loop biomass is less clear – it depends on what would have been done with the biomass if it had not been burned.

Reduced other emissions

Fossil fuel burning results in emissions, including SOx, NOx, and particulates. These emissions contribute to a variety of public health and environmental concerns. SOx and NOx are chief contributors to acid rain, as well as local air quality concerns such as smog. Sulfur is found in most coals, although coals vary considerably in their sulfur content. When coal is burned, the sulfur in the coal is emitted into the atmosphere as SO2 and other sulfate compounds (which as a group are termed SOx). NOx is formed when nitrogen in the atmosphere combines with oxygen – a process made possible by the high temperatures of fossil fuel combustion. According to the EPA, power plants are responsible for:

• 67 percent of all emissions of sulfur dioxide (SO2), the leading component of acid rain and fine particulates;
  
  
• 25 percent of all emissions of nitrogen oxides (NOx), a key component of ozone (smog), acid rain, and fine particulates;
  
  
• 34 percent of all emissions of mercury (Hg), a toxic heavy metal that is concentrated through the food chain.
  
  

What's often of interest, however, is the reduction in these emissions that would be achieved from greater use of renewables in a specific geographic area. The amount of emissions from fossil fuel burning varies considerably by the type of burning process (e.g. fluidized bed, cyclone, etc.) and the type of emissions control equipment in use. Fortunately, it is now possible to calculate the emissions from power plants in a specific area. The EPA's E-GRID database has detailed, plant-specific emissions data, including emissions in pounds per MWh, for individual plants. Utilities can use this database to calculate the emissions they can avoid from increased use of renewables.